1. Field of the Invention
This invention relates to a method and apparatus for controlling a force acting on a scanning probe of a scanning probe microscope (SPM), particularly an atomic force microscope (AFM).
2. Description of the Prior Art
Magnetically controlled SPMs have been noted in the literature for some time however one limitation to their widespread introduction is the deposition of magnetic material onto the force sensor. Prior art has included the glueing of small magnetic particles directly behind the tip and the deposition of thin magnetic films either on the back side or tip side of the force sensor. The problem with both the former methods is that it is time consuming, difficult and not reproduceable from sensor to sensor making any detailed calibration of force difficult. In the latter method the applied force tends to be much smaller, the reproducibility is still poor, the films often cause bending of the lever and have a tendency to peel in liquid.
It has already been suggested that forces can be directly applied to a scanned probe by the application of a magnetic moment via a current carrying coil located behind the lever to a magnetic film or particle magnetized along the length of the cantilever. This configuration can be problematic as the application of a torque along this axis of the probe can result in both normal and lateral motion of the probe tip which is undersireable, for example in storage and lithography applications.
In addition when used in a feedback scheme the application of a moment is not always effective in balancing the normal forces acting on the tip, particularly when an optical deflection detection system is used, as this measures the local slope of the cantilever rather than the absolute displacement of the tip. This is particularly problematic for soft cantilevers, where feedback based systems are of most use.
The application of an attractive force to a magnetic particle magnetised perpendicular to the cantilever in order to compensate for attractive forces between the tip and sample is also disclosed in U.S. Pat. No. 5,515,719. This is very limiting as it assumes that the force is always applied via a coil above the lever to compensate for an attractive tip-surface force. For most AFMs it is more convenient to locate the coil behind the sample as suggested by JP-P-A HEI 9-159682. It is also impossible to stabilize the lever against high force gradients unless both attractive and repulsive forces can be applied over high bandwidth.
Surface force apparatus (SFA) and atomic force microscopy (AFM) have been applied to the study of specific interactions between surfaces. In both of these instruments the detection of the force acting relies on the measurement of the deflection of a compliant spring or cantilever. Under such conditions the surface mounted on the compliant cantilever will jump into contact with the other surface if it experiences a force gradient exceeding the magnitude of the lever stiffness or due to thermal or mechanical noise at very small separations. This can be avoided by using a stiff cantilever, however this results in a degradation of the force sensitivity.
In the case of force spectroscopy, lever instabilities are detrimental to data interpretation because they cause discontinuities in the force data at short interaction ranges which are usually the regions of most interest. In addition, the kinetic energy associated with such instabilities can generate large contact stresses that may physically damage the surfaces, dominate the resulting contact size and prevent repeatability of the measurement. In the case of AFM any damage sustained by the tip drastically reduces the resolution for subsequent imaging.
To reduce this problem feedback methods have been implemented in an attempt to artificially enhance the stiffness of the force sensor by applying compensating forces directly to the cantilever. The vital component of understanding missing from the existing methods is that the lever must be controlled at its resonant frequency in order to alter its effective stiffness. This is relatively easy with SFA due to the low resonant frequency of the system, however with microfabricated AFM cantilevers the resonant frequencies are normally much higher making the requirements of the feedback electronics much more stringent. This marked difference between macroscopic levers and microfabricated levers in the field of feedback stabilization has not been pointed out in literature or patent disclosure to date.
What has been shown is the use of feedback schemes in the low frequency regime where it is possible to maintain a constant lever deflection in a limited bandwidth. This is effective in damping low frequency noise, however these systems do not alter the resonance frequency which clearly demonstrates that they do not alter the effective stiffness of the cantilever.
As it is the value of this effective stiffness in relation to the force gradient which determines the stability of the cantilever in high force gradients it is obvious that these systems will only work when it is solely low frequency noise which is causing the jump to contact. Further, it will not be possible to measure a force gradient greater than the stiffness of the lever.
The present invention has been proposed in view of the prior art drawbacks mentioned above and has as its object to provide a method and apparatus for controlling a force acting on a scanning probe of a cantilever, that can alter the effective stiffness of the cantilever and measure a force gradient greater than the stiffness of the cantilever.